3. Surveillance Programs

Issued: July 2013

Content last reviewed: July 2013

3.1 Surveillance Criteria

The average member of the population receives an annual radiation dose of 2 to 3.4 millisievert (mSv) from background radiation. This natural background radiation is made up of cosmic radiation, plus radiation from potassium, uranium and thorium that are present in the soil, and radon gas in houses.[R-2]

Ontario's current Drinking Water Quality Standards for radionuclide concentrations are also based on the 0.1 mSv dose limit.[R-5] Subsequently, the objective of the ORSP is to measure radionuclide concentrations which if received for a year would give a dose commitment of 0.1 mSv to a member of the public from either inhalation or ingestion.

3.2 Program Design

The radioactive elements associated with routine and accidental release from a CANDU reactor are well known and may follow a variety of pathways in entering the human system. To monitor these radionuclides in the environment requires specific programs be in place. Air, drinking water and food are each of particular concern. The ORSP is comprised of three principal surveillance programs and four special studies programs.

Surveillance programs:

air particulates

drinking water

tritium-in-air

Special Studies programs:

precipitation

milk

recreational surface waters

foodbasket – milk and vegetation

3.3 Site Selection & Program Summary

Five nuclear installations are currently in operation within or near Ontario's borders. Located north of Ottawa on the Ottawa River are the Chalk River Nuclear Laboratories. The Darlington and Pickering nuclear generating facilities are situated along the northern shore of Lake Ontario. The Pickering generating facility is on the eastern edge of the Greater-Toronto-Area while the Darlington generating facility is located east of Oshawa. The Bruce Power generating facility is situated on the north-east shore of Lake Huron. The Fermi-2 reactor, located 15 km south of Detroit, Michigan rounds out the list. This facility is near Windsor, another large urban centre.

Seven surveillance areas have been designated for use, and are listed below:

RPMS Surveillance Areas

Arthur – control station

Bruce – Bruce Power (formerly "Bruce Nuclear Power Development")

Darlington – Darlington Nuclear Generating Station

Essex – Fermi-II Nuclear Generating Station (located in Michigan)

Pickering – Pickering Nuclear Generating Stations

Ottawa – Chalk River Nuclear Laboratories

Toronto – water treatment plants

Sampling sites were located within each area using weighting factors for population density and distance from the facility. A complete listing of the monitoring sites and the programs carried out at these sites is provided in Table 2a and Table 2b. Maps 1 – 6 show the location of the individual monitoring sites within their respective surveillance areas.

The town of Arthur has been chosen as the control site for the ORSP as it is located mid-way between the Bruce and Pickering/Darlington Surveillance areas. Air samples collected here receive minimal possible exposure from these installations. Drinking water samples taken at Arthur, as measured by an independent laboratory, are extremely low in tritium and are used as a background blank by the RPMS.

Water treatment plants in the eastern part of the Toronto Surveillance Area are used to monitor drinking water for emissions from the Pickering nuclear generating facility. There are no water treatment plants in the Pickering Surveillance Area.

3.3.1 Enhanced Monitoring

Following the earthquake and tsunami which struck Japan on March 11, 2011 and subsequent failure of the Fukushima Daiishi nuclear power plant, a decision was made by the RPMS on March 16th to modify the routine sampling schedule at several air monitoring stations and precipitation collectors in the Pickering/Darlington region from monthly to weekly. This was done in order to provide background data in advance of any fallout from Japan.

As the situation in Japan continued to escalate, a second adjustment was made on April 20th to have samples of raw water and drinking water collected on a weekly basis from water treatment plants in the GTA. The frequency of milk sampling was also increased from the University of Guelph and in the Foodbasket program.

By the first week of June, the events in Japan had settled down and the decision was made to return to the normal sampling program. No significant change in activity had been detected in the results of sampling obtained during this period. The results of routine sampling and enhanced sampling will be presented together in this report.

3.4 Derived Survey Concentrations

Health Canada's guidelines for the radiological characteristics of drinking water, are based on a committed effective dose of 0.1 mSv from one year's consumption of drinking water.[R-3]

For the purpose of monitoring radionuclides the 0.1 mSv committed effective dose is expressed as a Derived Survey Concentration (DSC). The DSC has been determined for each of the radionuclides that are of interest in water using the Ontario Drinking-Water Quality Standards. In the absence of a formal exposure guideline for radionuclides in air the DSC has been determined using the associated DSC for water divided by 10. Together, the DSC for water and air are listed in Table 1.

3.5 Minimum Detectable Concentrations

The amount of radioactivity in a sample is calculated from the difference between the count rate with the sample in the counter and the "background" count rate when there is no sample in the counter. The background count rate is produced by the natural radiation present in the earth, fall-out from atomic bombs and cosmic radiation from space. Because of the random nature of radioactive decay, there is a statistical uncertainty associated with every measurement.

The minimum detectable activity (MDA) for a particular nuclide, analysis method, and counter is the smallest amount of radioactivity that will be distinguished from background in 95 out of 100 measurements. The minimum detectable concentration (MDC) is the minimum detectable activity expressed in terms of concentration per unit volume. Often, these terms are used interchangeably since the difference between MDA and MDC is in the conversion of the units.

In this report, the MDC is used as the majority of measurements are quoted in concentration units. For most nuclides, the MDC has been set at 10% of the DSC. For tritium in drinking water, the laboratory MDC is 5 Bq/L which is less than 0.1% of the DSC.

3.6 Data Handling For Unreported Data

Often, in the analysis of environmental sampling results are reported as less than the Minimum Detectable Concentration (MDC). This will produce a right-centred distribution of the results which are cut off below the MDC. It is important that this unreported or "missing" data be managed efficiently or an inaccurate inference may be drawn from the data.

There are several different strategies, sometimes referred to as "data imputation," which can be applied where the unreported values are replaced by a single value. This value may be chosen as the MDC, the MDC/2 or the MDC/√2. When this approach is used, however, the overall effect may bias the results if a large number of unreported values make up the data.

An alternative to data imputation is the use of regression estimation as a technique to replace the missing data. This approach is what has been used with the results of measurements from the water treatment plants that are made by the RPMS. In this method the variable with the missing values is treated as the dependent variable. The data is ranked from smallest to largest with those values that are below the MDC treated as the smallest values. The non-censored data is plotted on a scatter plot and should result in a straight line. Using this result a line of best fit can be drawn or calculated which will yield the regression equation. Through the use of this equation missing values can be predicted and the median and standard deviation of the data can be estimated.[R-4]